1. Field of the Invention
This invention relates to the analysis of complex mixtures of hydrocarbons. More particularly, it relates to the analysis of mixtures of paraffinic, olefinic and aromatic hydrocarbons by gas-liquid chromatography (GLC). It especially relates to the chromatographic analysis of gasolines.
2. Description of the Prior Art
In gas-liquid chromatography, a mobile phase, such as a gaseous stream of nitrogen or helium containing a mixture of solutes is passed over a stationary phase of a nonvolatile liquid which is evenly distributed as a thin layer on a noninteracting solid support. The stationary phase is most conveniently provided in a column. Each solute is translated down the column by the mobile phase alternately distributing itself between the two phases as it moves. The species become separated because the individual components travel at different rates through the column depending on their affinity for the gas and liquid phases. This affinity is referred to as the partition coefficient and is the ratio of the concentration of a species in the stationary (liquid) phase to its concentration in the mobile (gas) phase. The mixture is separated into its components, depending on the nature of the mixture and the immobile phase in the column, and passes from the column for analysis by a detector, typically a thermal conductivity detector or a flame ionization detector. The detector indicates and measures the amount of separated components in the carrier gas. The detector response is usually a series of peaks recorded as a function of time and constitutes a gas chromatogram. The time required for a component to pass through the column, or its retention time, is a qualitative factor, while the detector response, which can be measured as peak height or area can be related to concentration. The distance between two peaks indicating two components increases in proportion to the distance traveled and the width of the peak increases as the square root of the distance. Where the peaks are overlapping or resolution in the chromatographic column is insufficient between components with similar properties it is often useful to collect the effluent from the first column and inject it into a second chromatographic column employing a liquid phase with a different selectivity to provide more suitable conditions for the desired separation and analysis.
Analysis of gasoline by means of chromatography is known in the art. The procedures employed heretofore are often complex but nonetheless may be limited in the completeness of the analysis.
Schulz et al utilized several systems for analyzing olefin-containing gasolines so as to identify the paraffins, olefins and aromatics by gas chromatography. In one procedure, the olefins plus aromatics were reversibly absorbed in a pre-column of AgNO.sub.3 on Sterchamol and the saturated gasoline portion was subjected to capillary gas chromatography. After the chromatogram of the paraffins was made, the pre-column was desorbed with a carrier gas to produce a chromatogram of the olefins and aromatics. However, this method did not permit complete identification where olefin isomers were very numerous. In another procedure used by Schulz et al, a capillary chromatogram was taken of the paraffins in a gasoline after the unsaturates (olefins and aromatics) were removed by sulfuric acid scrubbing in a pre-column. This analytical procedure was completed by use of another pre-column where the olefins, but not the aromatics, were selectively hydrogenated and the hydrogenated sample was analyzed to produce a second capillary chromatogram. Although this latter procedure yields a very accurate and complete analysis, it is very complex. (See 27 Erdoel Kohle, Erdgas 25, Petrochem, Brennst-Chem. 345-52 (1974).)
Block et al. employed a chromatographic analysis to determine the composition of a methanol-derived gasoline which had a maximum carbon number of C.sub.11. In this procedure, the aromatics were reversibly absorbed in a pre-column followed by chromatographic analysis of the saturates and olefins remaining in the sample. The olefins were then removed from the sample in an absorber and the remaining saturates were resolved and analyzed in a chromatographic column. Following desorption by a carrier gas, the aromatics were chromatographically analyzed. The aromatic pre-cutter column was a wall coated open tubular (WCOT) column coated with a polar liquid phase of cyanopropyl phenyl silicone. The olefin absorber was mecuric perchlorate-perchloric acid (MP-PA) dispersed in a packed column. This absorber requires that the gas feed to the column have a precisely controlled water content. The two resolving columns employed were support coated open tubular (SCOT) columns using squalene as the coating in the aromatics column and squalane in the saturates and olefin resolving column. Three flame ionization detectors permitted simultaneous analysis of aromatics, saturates plus olefins and saturates. Although this procedure produced a detailed analysis, for the components analyzed, hydrocarbons heavier than C.sub.10 -C.sub.11 were not analyzed. Further, the apparatus is extremely complex since it requires four ovens operating at different temperatures, two temperature programs, three flame ionization detectors and an effluent splitter for controlling temperature and pressure. In addition, the reversible aromatic absorber has a selectivity of C.sub.9 -C.sub.11 which limits its effectiveness when analyzing higher boiling gasolines. Also, water vapor pressures can be a problem here. Traces of water vapor cause deterioration of the liquid phase aromatics absorber while the precise control required for the olefin absorber makes reproducibility a serious problem. It was for this latter reason that the ASTM abandoned development of this MP-PA column as a standard test. Overactivity of the MP-PA column can also undesirabily absorb branched paraffins and aromatics. (See, 15 Chroma. Science 504-12 (1977).)
British Pat. No. 1,146,250 discloses a method of gas-liquid chromatography analysis of hydrocarbon mixtures utilizing columns of different selectivity connected in series wherein one group of components, for example, aromatics, is separated in a first column containing a polar immobile liquid phase and another group, for example, paraffins, naphthenes and olefins, passes through the first column with a different speed and, without undergoing appreciable separation, finds more suitable conditions for separation in a second column which contains a non-polar immobile liquid phase. In one embodiment, the effluent from a preceding column is collected by freezing out this fraction in a U-tube packed with a filler material. This fraction is then injected into a following column by raising the temperature of the U-tube while the carrier gas is passed therethrough. By utilizing a number of columns containing polar and non-polar immobile liquid phases and by collecting the several effluents for injection in a following column, the mixture is separated into its individual components for analysis by flame ionization detectors. Complete resolution and identification of a complex mixture of hydrocarbons required four chromatographic columns, two with a polar immobile liquid phase and two with a non-polar immobile liquid phase, and four U-tubes necessitating at least four freezing and heating operations resulting in the production of four chromatograms for a quantative analysis of a naphtha sample.
It is an object of this invention to chromatographically analyze the paraffin, olefin and aromatic components of a gasoline containing up to at least C.sub.13 hydrocarbons with a minimum of equipment and analytical operations.